Future generations of optoelectronic devices for telecommunications, information storage, external generation photoelectric devices, optical switching, and signal processing are predicated to a large degree on the development of materials with large nonlinear optical (NLO) responses. Toward this end, considerable effort has been directed to elucidating the chromophore design elements that correlate most closely with large first-order hyperpolarizabilities. This is due to the fact that an NLO chromophore with a high molecular first hyperpolarizability would be a highly desirable candidate for incorporation into ordered polymeric systems, and thus serve as the basis for macroscopic materials for frequency doubling as well as optoelectronic devices that function as waveguide switches, modulators, filters, and polarization transformers.
Design, modification, and further fine-tuning of the magnitude of the molecular first hyperpolarizability (β(0)) of a given chromophore has generally been thought of in the context of Oudar's two-state model:(β(0))∝(μee−μgg)μge2/Ege2  (1)where g and e represent the indices of the ground and charge transfer (CT) excited states, respectively; μ is the dipole matrix element; and E is the transition energy.
Most NLO chromophores are composed of an electron donor (D) and an electron acceptor (A), the molecular entities chiefly involved in charge redistribution, as well as a bridge (i.e., the molecular scaffolding that links the D and A portions of the chromophore). To date, the design of chromophores with good second-order nonlinear properties has focused primarily on engineering: (i) the electronic nature of the D and A, and (ii) the conjugation length of the bridge. The former controls D-A mixing with respect to a specific bridge, while the latter plays a role in modulating D-A electronic coupling and also determines the magnitude of the change in dipole moment. Per Equation (1), increasing the bridge conjugation length increases the magnitude of the change of dipole moment, while concomitantly diminishing the square of the dipole matrix element and increasing the square of the CT transition energy; the latter two effects have their genesis in the fact that increased bridge lengths attenuate D-A electronic coupling. Maximizing β(0) thus involves an interplay between three parameters that do not necessarily simultaneously attain their optimal value for a particular molecular structure (D, A, bridge).
Most of the chromophores that have been studied to date for their second-order nonlinear properties can be classified as D-A systems in which the molecular bridge is based either on ethene, phenylene, ethyne, small-ring heteroaromatic, styrene building blocks, or a combination of two or more of these simple units. Although a variety of different organic media have been utilized as D-A bridging moieties, comparatively little attention has been paid to how the details of the bridge topology and electronic structure impact the chromophore second-order NLO response, particularly when viewed alongside the body of literature describing how D-A electronic properties and bridge length modulate the molecular first hyperpolarizability.
Engineering of bridge electronics and topology has been a primary focus in developing new classes of NLO chromophores with exceptional photophysical properties. A basic criterion is that the bridge should be much more polarizable than the simple polyene, polyyne, polyphenylene, and polyheteroaromatic structures that have been most commonly used. Ideally, the bridge-localized excited state should dramatically alter D-A electronic coupling relative to the coupling the ground-state bridge provides.
One approach to enabling such differential ground- and excited-state coupling is to choose a D-A bridging motif that is capable of accessing a resonance form in its excited state that is unattainable for the ground-state structure. Such an excited-state resonance structure would be optimal if it produced a large transition dipole oriented directly along the D-to-A molecular charge transfer axis. A designed excited state with these properties would facilitate large molecular first hyperpolarizabilities since the magnitude of the change in dipole moment would not be held ransom by significant diminution of the oscillator strength of the CT transition or an increase in the transition energy at relatively large D-A distances, since a high oscillator strength, bridge-centered transition would now directly couple D to A. Presumably, if the orientation and dipolar nature of the bridge-centered CT transition could be maintained over a long range in such a system, μee−μgg and μge2 would simultaneously increase with augmented bridge lengths while concomitantly maintaining or slightly reducing Ege2.
Exemplifying some of the desired properties of NLO chromophores discussed above are the so-called push-pull arylethynyl porphyrin systems described in U.S. Pat. No. 5,783,306; LeCours, et al., J. Am. Chem. Soc., 1996, 118, 1497; Priyadarshy, et al., J. Am. Chem. Soc., 1996, 118, 1504; LeCours, et al., J. Am. Chem. Soc., 1997, 119, 12578; and Karki, et al., J. Am. Chem. Soc., 1998, 120, 2606. These compounds, which contain organic donor/acceptor groups connected through a porphyrin-based bridging system, exhibit remarkably large molecular first-order hyperpolarizabilities. In addition to very large molecular first hyperpolarizabilities, chromophores for optoelectronic applications often require thermal stabilities in excess of 200° C. Thus, new compounds are needed that show both large hyperpolarizabilities and greater thermostability. The compounds, compositions, devices, and methods of the present invention address these and other needs.